The objective of this paper was to summarize the physical and chemical knowledge of the transferrin molecule. The ample data concerning transferrin were presented. Considerable attention was given to the microheterogeneity of the transferrin molecule--the polymorphism, differences of the binding site for iron and possible donation of iron to the cell, number of carbohydrate antennae, degree of sialyzation--in connection to the possible link between microheterogeneity and physiological and pathophysiological status of the organism.